We report the electrocatalytic activity of a new class of superparamagnetic nanoparticles, graphene-oxide-loaded iron oxide (GO/IO hybrid material), towards the reduction of ruthenium hexaammine(III) chloride (Ru(NH3)6]3+, RuHex). Leveraging the electrocatalytic activity of the GO/IO hybrid material and the signal enhancement capacity of [Ru(NH3)6]3+/[Fe(CN)6]3 in an electrocatalytic cycle, an ultrasensitive and specific electrochemical sensor was developed for the detection of cancerrelated microRNA (miRNA). Using the direct affinity interaction between RNA and graphene oxide, magnetically isolated and purified target miRNA were directly adsorbed onto a screenprinted electrode modified with the GO/IO hybrid material. The detection was enabled by chronocoulometric (CC) readout of charge-compensating [Ru(NH3)6]3+ followed by an enhancement in CC charge display through the Ru(NH3)6]3+ /[Fe(CN)6]3 system. We demonstrate an excellent limit of detection of 1.0 fM by accurately detecting miR-21 in synthetic samples and showcase its clinical utility in ovarian cancer cell lines with high sensitivity (ten cells) and good reproducibility (%RSD= <5%, for n=3).